Once you upgrade your tower, it allows for a garrison. You can add heroes to the tower at that point. You can take them out, put them back in, or have no garrison. It gives your tower special powers so to speak.

Get your heroes into the dungeon and level the shite out of them. Upgrade your troops as often as possible. Don't focus upgrading on the first 2 category of troops, focus on the last 2. Look back through this thread. There is lots of useful info.